Viola Birss

Chromium Poisoning of LSM-YSZ SOFC Cathodes I. Detailed Study of the Distribution of Chromium Species at a Porous, Single-Phase Cathode

Detailed chemical and surface structure analyses are presented for an 8 mol % yttria-stabilized zirconia (YSZ) wafer, partially covered with a thin, porous (La 0.8 Sr 0.2 ) 0.98 MnO 3 (LSM) film electrode, after being cathodically poisoned in the presence of a chromia source at 800°C. Under the conditions used, the newly deposited Cr-containing compounds were distributed not only in and around the electroactive LSM-YSZ-air triple-phase boundary (TPB) interface region, but also extending ca. 500 μm from the edge of the LSM film onto the YSZ surface. The distribution and structure of the Cr species on the YSZ surface was directly correlated with variations in the cathodic polarization of the half-cell for a solid oxide fuel cell (SOFC). Such structuring was not anticipated, considering that the electrochemical activity is thought to occur only at the LSM-YSZ-air TPB. Evidence presented here suggests that Cr 2 O 3 plays an important role in extending the electroactive LSM-YSZ-air TPB out onto the YSZ surface by effectively creating a new Cr 2 O 3 -YSZ-air TPB region.

Platinum oxide film formation-reduction : an in-situ mass measurement study

Abstract The growth and reduction of oxide films at Pt was studied using the quartz crystal microbalance technique. The compact α-oxide was shown to be anhydrous in nature (either PtO or PtO 2 ). When the potential was extended into the hydrogen adsorption region, the electrode mass was seen to decrease, but then stabilized once the hydrogen evolution reaction commenced. These results, as well as the effect of the addition of chloride and ferrous ions on the mass measurements, have been interpreted in terms of ion adsorption—desorption processes. The mass change during the reduction of thin hydrous β-oxide films was also monitored and yielded a mass-to-charge ratio consistent with a PtO(OH) 2 film composition. Other mass data indicated that Pt dissolution can occur with repeated β-oxide growth—reduction cycles and that some Pt may redeposit, given sufficient time at negative potentials.

Electrochemical Studies of AC/DC Anodized Mg Alloy in NaCl Solution

“© The Electrochemical Society, Inc. 2004. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Journal of the Electrochemical Society, 2004, Vol. 151(3).”

A Kinetic Study of the Oxygen Reduction Reaction at LaSrMnO3 - YSZ Composite Electrodes

The primary focus of this paper is on the establishment of reliable methods for the determination of the mechanism and kinetics of the oxygen reduction reaction (ORR) at solid oxide fuel cell cathodes consisting of lanthanum strontium manganite [(La 0 . 8 Sr 0 . 2 ) 0 . 9 8 MnO 3 , LSM] in a 50 vol % mixture with yttria-stabilized zirconia electrolyte (LSM-YSZ composite). Techniques used include half-cell cyclic voltammetry and electrochemical impedance spectroscopy (EIS) methods in a variable po 2 atmosphere at temperatures ranging from 600 to 900°C. The exchange current densities for the ORR, determined both from the low and high field cyclic voltammetry data and from the charge-transfer resistance from EIS data, are shown to agree closely, yielding an apparent activation energy of ca. 120 kJ/mol for the ORR at these composite cathodes. No evidence for diffusion-controlled reactions is seen under the conditions of our work. In this paper we also show the theoretically predicted impact of temperature on the Tafel slope, as well as on the potential range over which the low- and high-field approximations, are valid.

A MODEL FOR ANODIC HYDROUS OXIDE GROWTH AT IRIDIUM

Abstract A detailed investigation of the electrochemistry of Ir in 0.5 M H 2 SO 4 has been used as an experimental basis for a model for oxide growth at Ir. It appears that a compact oxide (probably IrO 2 ) is formed initially. At potentials above + 1.2 V vs. RHE, the outer monolayer of this compact oxide is oxidised and becomes hydrated. The hydrated surface layer inhibits further oxidation of the compact oxide and therefore only one monolayer of hydrous oxide can be formed at constant potential. To obtain more hydrous oxide than this, the compact oxide must be continually reduced to Ir metal and reformed, by cycling of the potential. On each cycle, the hydrated surface layer of the compact oxide remains after reduction of the compact oxide. Thus, this material accumulates as a hydrous oxide layer.

Characterization of Oxide Films Formed on Mg-Based WE43 Alloy Using AC/DC Anodization in Silicate Solutions

The characteristics of the oxide film formed on a Mg-based WE43 alloy using ac/dc anodization techniques in an alkaline silicate solution have been investigated using scanning electron microscopy and Rutherford back-scattering spectroscopy, and the corrosion resistance of these films has been determined using the ac impedance technique. The oxide film was found to be composed of MgO, Mg(OH) 2 , MgF 2 , and SiO 2 , and consisted of an inner barrier and an outer porous oxide film, both of which increased linearly in thickness with the applied anodization voltage. High applied voltages and current densities also led to the formation of large pores, having diameters greater than 6 μm, and the partial sealing of small pores in the outer layer. Allowing the current to decay for a period of time in the later stages of anodization led to an increase in the thickness of both the barrier and porous films and further sealing of the film pores. A correlation of the oxide corrosion resistance with the barrier film thickness is also demonstrated.

Electron Microscopy Study of Formation of Thick Oxide Films on Ir and Ru Electrodes

“© The Electrochemical Society, Inc. 1984. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Journal of the Electrochemical Society, 1984, Vol. 131(7).”

Multi-technique study of the anodic degradation of polyaniline films

Abstract Polyaniline (PANI) films, formed by potential cycling methods on Au electrodes in sulfuric acid solutions, were anodically degraded with time at ca. 1 V versus RHE. Ellipsometric and cyclic voltammetric data suggest that colored species are formed within the film, perhaps containing quinone endgroups, and the bulk of the film dissolves uniformly creating greater porosity. This model of film degradation has been supported by transmission electron microscopy (TEM) studies of cross-sectional samples of PANI films. Both TEM and ellipsometry also showed that the PANI film is not delaminated, and is very resistant to anodic dissolution/degradation. Concurrent in situ mass measurements suggest that, in the early stages of film degradation, some regions of the film become electrochemically inactive, although the nature of the charge compensation mechanism of the PANI redox reaction remains unaltered. When no further film degradation can be accomplished, the mass data show that a substantial quantity of largely inactive film material is still present on the electrode surface. Furthermore, the change in the redox potential and the invariant charge compensation stoichiometry suggests that, although the final degraded film is a different material from the newly formed PANI film, only those portions of the film that have PANI characteristics are electrochemically active.

Glucose detection based on electrochemically formed Ir oxide films

Abstract Ir oxide (IrOx) films, known to be nanoporous, electronically conducting and biocompatible, were formed anodically on polycrystalline Ir substrates in neutral solutions and were used to immobilize glucose oxidase (GOx) using two different methods. The resulting electrodes were found to generate H 2 O 2 in the presence of glucose and oxygen, yielding an excellent response to glucose (up to 70 mM) by the oxidation of H 2 O 2 in both stirred and non-stirred neutral solutions. It was found that, while the preferred method of immobilization involves the growth of IrOx films in a ca. 130–160 U ml −1 GOx solution, these electrodes displayed relatively large K ′ m values (up to 190 mM), reflecting restricted diffusion of glucose inside these films. The thicker the film on the electrode, the more stable the electrode response, especially to solution agitation, but the larger were the K ′ m values. SEM examination of these electrodes showed that GOx deposits in the form of small clusters, with the more stable response obtained when these clusters are more uniformly distributed. These amperometric IrOx/GOx glucose sensors were found to be free of interference from ascorbic acid, uric acid and paracetamol, at their physiological concentrations.

A multi-technique study of compact and hydrous Au oxide growth in 0.1 M sulfuric acid solutions

Abstract The growth and reduction of compact (α-) and overlying hydrous (β-) oxide films on polycrystalline Au electrodes in aqueous 0.1 M H 2 SO 4 solutions have been investigated using potentiostatic, cyclic voltammetry, ellipsometric and quartz crystal microbalance (QCMB) techniques. All α-oxide films, formed with time at constant potentials up to 2.6 V, or by multicycling of the potential, are non-hydrated in nature, even when covered by a thick β-oxide film. The α-oxide film composition is suggested to be AuO below 1.5 V, and a mixture of AuO+Au 2 O 3 at potentials above this, becoming predominantly Au 2 O 3 at very high potentials. Up to three monolayers of Au 2 O 3 can be formed. When formed at constant potential, the β-oxide film becomes increasingly hydrated as it thickens with time of growth, with a mass to charge ratio and refractive index consistent with Au 2 O 3 ·H 2 O and later with Au 2 O 3 ·2H 2 O. In contrast, the β-oxide film formed by multicycling has a higher mass overall, and becomes less hydrated as it thickens with time, with a mass and refractive index consistent with Au 2 O 3 ·10H 2 O at short times, ranging to Au 2 O 3 ·2H 2 O as the film thickens.